Diamond nuclear radiation detector

ABSTRACT

A diamond nuclear radiation detector comprising a diamond crystal plate and contacts on the opposite sides of said plate adapted for the application of an electric field to said diamond crystal plate. The contact on the side of the crystal plate to be irradiated in the course of detecting nuclear radiation is constituted as a blocking contact in relation to charge carriers, whereas the opposite contact is constituted of a meterial capable, in conjunction with the diamond plate, of injecting the charge carriers under the influence of the electric field. The thickness of the diamond crystal plate between the contacts does not exceed the maximum drift length of the charge carriers created by the detected nuclear radiation in the diamond crystal plate under the influence of the applied electric field corresponding to the maximum drift rate.

United States Patent I I I t [151 3,665,193

Kozlov et a]. y Y 1 May 23, 1972 s41 DIAMOND NUCLEAR RADIATIQ 2,765,38510/1956 Thompson ..2so/s3.3 x DETECTOR 3,113,220 12/1963 Goulding et al.....250/83.3 X y 3,212,940 10/1965 Blankenship ..250/83.3 UK [72]Inventors: Stanislav Fedorovich Kozlov; Elena Alexandrovna Konorova,both of Moscow, Primary Examiner James Lawrence Assistant ExaminerMortonJ. Frome [73] Assignee: Ordena Lenina Fizichesky Institut lmeni P.Aflomey-watefs, Rodifi, Schwartz & Nissen N. Lebedeva Leninsky Prospekt,Moscow,

U.S.S.R. [57] ABSTRACT [22] Filed: Mar. 28, 1968 A diamond nuclearradiation detector comprising a diamond crystal plate and contacts onthe opposite sides of said plate [21 1 Appl' 716353 adapted for theapplication of an electric field to said diamond I crystal plate. Thecontact on the side of the crystal plate to be [30] Foreign ApplicationPriorityData irradiated in the course of detecting nuclear radiation isconstituted as a blocking contact in relation to charge carriers, Mar.29, l l whereas the opp Contact is constituted ofa material p ble, inconjunction with the diamond plate, of injecting the [52] U.S. Cl...250/83 R, 3l7/234Z charge carriers under the influence of the chem:fielcL The [51] lnt.Cl

[ Field ch U202 thickness of the diamond crystal plate between thecontacts 833; 317/234 does not exceed the maximum drift length of thecharge carriers created by the detected nuclear radiation in the diamond[56] Reerences Clted crystal plate under the influence of the appliedelectric field UNITED STATES PATENTS I corresponding to the maximumdrift rate.

2,760,078 8/1956 Yoamans 250/833 v 14 Clairm, 2 Drawing Figures DIAMONDNUCLEAR RADIATION DETECTOR The present invention relates to nuclearradiation detectors and methods of manufacturing said detectors.

There are known nuclear radiation detectors consisting of a crystal ofnatural .diamond with a lower nitrogen content (the nitrogenconcentration is usually less than atoms cm provided with electriccontacts. When applying a potential difference across the diamond andirradiating it with nuclear particles from any side, current pulses areinduced inside the crystal. These current pulses produce an externalcircuit voltage pulses which are amplified and counted by appropriateapparatus.

This behavior is shown only by a small number of diamond crystals andthe counting properties of such detectors are diverse anduncontrollable. Such detectors have poor counting efficiency and lowenergy resolving power, and operate with incomplete collection of thecharge carriers created in the crystal by incident nuclear particles. Inaddition, electric polarization occurs in these crystals, since theresistivity of said crystals is high. As a result, their countingproperties deteriorate under irradiation. Known methods of removingpolarization by heating or illumination with light of appropriatewavelengths are inconvenient and ineffective. For these reasons, thedetectors on the basis of diamond have not found wide practicalapplication.

It is an object of the present invention to produce the diamond detectorthat would operate at room and higher temperatures and possess goodcounting efficiency, complete charge collection and high energyresolution, as well as constancy of its properties under prolongedirradiation, said detector being made from a high resistivity diamondcrystal and, hence, operating without increasing the noise level.

Another object of the invention is to develop a method of manufacturingsaid detector.

In the accomplishment of said and other objects of the invention, in thedetector consisting of a diamond crystal plate with two electriccontacts located at its opposite sides across which plate a potentialdifference is applied, according to the invention, the thickness of theoperating range of the plate between the contacts does not exceed thedistance traveled under the influence of the applied electric field bythe charge carriers created by nuclear radiation in the diamond crystal.The contact through which the incident nuclear particles penetrate intothe crystal is made blocking in relation to the charge carriers, whilethe opposite contact is made from a material capable, in conjunctionwith diamond, of injecting the charge carriers into the crystal underthe influence of the electric field.

If the detector is made from diamond in which the distance traveled byelectrons is longer than that travelled by holes, the contact on theunirradiated side of the plate should inject holes and the positivepotential is applied to this contact. If the detector is made fromdiamond in which the distance traveled by electrons is less than thattraveled by holes, the contact on the unirradiated side of the plateshould inject electrons and the negative potential is applied to thiscontact.

In case the condition of complete charge collection is observed providedthat the thickness of the diamond crystal plate is low, a recess is madein the thick plate of the crystal with a view to increasing themechanical strength of the detector, the thickness of the bottom of saidrecess being equal to the distance traveled by the charge carriers.

As the experiments have shown, silver, gold, platinum and graphite maybe used as a material insuring in conjunction with diamond a contactinjecting holes. Such a contact may also be provided by the surfacelayer of the diamond crystal plate doped with aluminum or boron.

Graphite may serve as a material insuring in conjunction with thediamond a contact'injecting electrons. This contact may. also beprovided by the surface layer of the diamond crystal plate doped withphosphorus, lithium or carbon.

Used as a material providing a blocking contact may be gold, silver orplatinum. Such a contact may also be provided by the graphitized surfaceof the diamond crystal plate and by doping the surface layer of theplate with boron, aluminum,

phosphorus, lithium and carbon. The formation of the blocking andinjecting contacts is secured not only by using said materials, but alsowith the aid of applying to it the potential of appropriate polarity, aswell as owing to the damage of the surface crystalline structure of theplate, such as in'the case of graphitization or doping.

The present detector may be manufactured by the method wherein,according to theinvention, a plate is cut off a diamond crystal, thethickness of the plate being equal to the distance traveled by thecharge carriers in the crystal. With a view to prolonging the lifetimeof the carriers, said plate is annealed in vacuum at l,O00 to l,300 C.Prior to forming contacts the annealed plate of the diamond crystal isetched by heating in oxygen-containing medium in order to reduce therate of surface recombination of the charge carriers, if necessary.

For forming the blocking and injecting contacts both sides of thecrystal plate are covered with paint of silver, gold or platinum, andthe plate is heated to a temperature of 500 to 700 C. The plate is heldat this temperature for 2 to 3 hours in order to burn the metal into theplate. v

Blocking and injecting contacts can be formed by applying to both sidesof the crystal plate a solution of gold,.silver or platinum salts and byheating the plate to a temperature of 500 to 700 C for several minutesin order to restore the metal.

For forming a graphite contact for injecting both electrons and holes,one side of the diamond crystal plate is covered with a colloidalgraphite suspension, and the plate is heated in vacuum to a temperatureof 500 to 600 C for about 3 hours.

A blocking contact is obtained by evaporating a film of gold, silver orplatinum over one side of the plate. In some cases, for accomplishing ablocking contact the diamond crystal plate is graphitized by heating invacuum of 0.1 torr for about 30 min at a temperature of 1,000 to 1,300C.

For a better understanding of the invention, presented hereinbelow isthe description of an exemplary embodiment thereof with reference to theaccompanying drawings, wherein:

FIG. 1 shows the detector in accordance with the invention; and

FIG. 2 shows the detector of the invention consisting of a diamondcrystal plate with a recess, provided with contacts.

The detector of the invention (FIG. 1) consists of a diamond crystalplate 1 on whose opposite sides the contacts 2 and 3 are provided.Saidplate l is made from a diamond in which the distance traveled by theelectrons is longer than that traveled by the holes. Therefore, thecontact 2 is made from silver which, in conjunction with the diamond andunder the influence of the positive potential applied to it, injectsholes into the diamond crystal. The opposite contact 3 made from gold isa blocking contact in relation to the charge carriers when the negativepotential is applied to it.

Nuclear radiation entering the detector from the side of the blockingcontact 3 causes ionization inside the diamond crystal. The resultingcharge carriers, i.e. the electrons and holes, move to the contactsunder the influence of the applied field, the electrons moving to thecontact 2, and the holes traveling to the contact 3. The thickness ofthe crystal plate 1 does not exceed the distance traveled by the chargecarriers in the diamond crystal under the influence of the appliedfield. In the case of detectors, operating with complete chargecollection, high energy resolution and good counting efficiency, thefollowing condition should be observed,

, d 5 'r E, where ,u is the mobility of the charge carriers, 1- is thelifetime of the charge carriers, E is the applied field strength, 5 isthe distance traveled by the charge carriers under the influence of theapplied field, and d is the thickness of the diamond crystal plate.

It is well known that in diamonds, in which the nitrogen concentrationdetermined by optical absorption at a wavelength of 7.8 u is less thanatoms cm, the mobility of electrons is about 2,000 cm v sec' at roomtemperature, while the mobility of holes is about 1,500 cm V' sec". Inthe purest diamond crystals the lifetime of the charge carriers rangesfrom 10 to 10 sec. The experiments have shown that the mobility ofelectrons and holes in the diamond at high electric fields decreases asthe field is increased, at first proportional to E, then proportional toE beginning with a field strength of IOV cm for electrons at roomtemperature. Thus, the drift velocity ,uE saturates at high fields andits limit for electrons is 10 cm sec at room temperature. Consequently,at the lifetime of the charge carriers of IO sec the diamond crystalplate for the detector operating with complete charge collection has theoptimum thickness of 0.2 to 0.3 mm. At the shorter lifetime of thecharge carriers the diamond crystal plate should be thinner and itsthickness is estimated in accordance with the above equation.

On their movement to the contact 2 some electrons are trapped by trapsalways present in the crystal. As a result, the diamond crystal platepolarizes. The injecting contact 2 is designed to remove saidpolarization. Since deep traps are present in the diamond, the injectioncurrents from the contact 2 are limited by space charge accumulated bysaid traps. Thus, the injection currents do not induce significantconductivity and, consequently, noise. However, when field and chargeequilibrium inside the crystal is disturbed due to polarization createdby incident nuclear radiation, charge emission from the contact 2restores the initial steady state of the crystal. Since the higher fieldstrength within the ionization zone favors the reduction of losses inthe electron-hole plasma when using the detector for counting thenuclear particles with low penetration, the blocking contact 3 should belocated on the irradiated side of the plate I.

The charge carriers, holes, which move to the blocking contact 3 underthe influence of the applied field may also be trapped. In this case,however, the trapped holes are in the ionization zone and can beneutralized by the charge carriers of opposite sign, i.e. by electrons.

Thus, the present detector operates with complete charge collection anddoes not polarize under prolonged irradiation due to the thickness ofthe diamond crystal plate which does not exceed the distance traveled bythe charge carriers and the appropriate contact system is provided.

In like manner, the detector can be manufactured from the diamondcrystal in which the distance traveled by electrons is less than thattraveled by holes. The difference is that the contact on the irradiatedside is a blocking contact in relation to electrons and the positivepotential is applied to it, whereas the opposite contact injectselectrons and the negative potential is applied to it.

The detector shown in FIG. 2 is made from a diamond crystal plate whosethickness is considerably larger than the distance traveled by thecharge carriers. A recess is made therefore in the crystal plate, thethickness of the bottom of said recess not exceeding the distancetraveled by the charge carriers. This detector operates in the same wayas the detector described above, has greater mechanical strength owingto the thickened peripheral area and is more convenient in handling.

The selected crystals are cut in plates 0.1 to 0.3 mm thick. Aftercutting, the plates are placed in 10 torr vacuum and annealed for 6 to 8hours at a temperature ranging from l,0O0 to 1,300 C. In some cases, thethermal treatment increases the lifetime of the charge carriersconsiderably,

In the annealed plates the lifetime of the charge carriers is once moreestimated by using the same technique, but at wavelengths of 225 and 220mg" The edge absorption begins in the diamond at these wavelengths. Theabsorption coefficients are 20 and 1,000 cm", respectively. The meanlifetime of the charge carriers throughout the crystal is estimated onthe value of photocurrent at 225 mp, whereas the effect of surfacerecombination upon the value of photoconductivity is estimated at awavelength of 220 my. (the depth of light penetration is about 20p).

The plates with the mean lifetime throughout the crystal of the order of10' sec and higher are selected for further treatment.

In some cases, surface recombination is significant. The reduction ofthe rate of surface recombination is obtained by oxygen etching thespecimens in the atmosphere for several minutes at a temperature of 800to 900 C. If the thickness of the diamond plate after cutting, annealingand etching is larger than necessary for the operation of the detectorwith complete charge collection, the plate is thinned to the desiredthickness, for example, by polishing, grinding or etching.

After mechanical treatment the crystal plate is subjected to thermaltreatment as it has been described hereinabove and to etching.

Then, contacts are applied to the prepared plate. The simplest method ofobtaining a contact for injecting holes on one The diamond detectordescribed hereinabove is made from natural diamond with a nitrogencontent less than 10 atoms cm'. The selection of crystals for makingdetectors is based on the estimation of the mean lifetime of the chargecarriers throughout the crystal by measuring a photocurrent value at awavelength of 250 mp. The photoconductivity at 250 mp, is imperfectionphotoconductivity and the optical absorption coefficient at 250 my. hasa value ranging from 5 to 15 cm. Under these conditions one mayconsider, with an accuracy of 50 percent, that the light is completelyabsorbed in said crystals. Then, the mean lifetime of the chargecarriers throughout the crystal can be estimated on the value ofphotocurrent, without measuring the absorption coefficient. The crystalswith the mean lifetime longer than 10 see are selected for making thedetectors.

side of the plate consists in applying silver paint with subsequentburning it into the plate in the atmosphere at about 600 C during 2 to 3hours. A blocking contact is formed by evaporating a film of gold overthe opposite side of the plate in vacuum at room temperature.

In like manner, an injecting contact can be formed on one side of theplate by burning into the plate gold or platinum from paint. A contactfor injecting holes is also formed by restoring platinum, silver or goldfrom a solution of their salts by heating the diamond crystal platecovered with said solution to a temperature in the range from 500 to 700C for several minutes.

For the formation of a graphite contact injecting both electrons andholes a colloidal graphite suspension, such as Aquadag, is applied toone side of the diamond crystal plate and the plate is heated to atemperature ranging from 500 to 600 C in vacuum for about 3 hours. Then,over the opposite side of the plate a film of gold is evaporated forfonning a blocking contact.

In some cases, a blocking contact has been obtained by graphitizing thediamond crystal plate by heating in the temperature range from l,000 to1,300 C in vacuum of 0.1 torr for about 30 min. Then, the resultinggraphite layer has been removed from one side of the plate. A film of acolloidal graphite suspension has been applied to this side. Then theplate has been heated in vacuum at a temperature ranging from 500 to 600C for obtaining an injecting contact. In like manner, an injectingcontact can be obtained after the removal of said layer by burning intothe plate silver from paint as described above. The present diamonddetector for nuclear radiations has a number of advantages. It candetect the nuclear particles with the range up to 2X10 cm and operatesat room and higher temperatures. In addition, it possesses high energyresolving power of 7 percent at room temperature and counting efficiencyof percent. The detector operates with complete charge collection anddoes not polarize under prolonged irradiation.

We claim:

1. A diamond nuclear radiation detector comprising a diamond crystalplate having opposite sides one of which is irradiated; contacts on theirradiated and opposite sides of said plate adapted for applying apotential difference thereacross when detecting nuclear radiations, oneof the contacts being adapted for the application of a positivepotential, whereas the opposite contact is adapted for the applicationof a negative potential, said contacts and said plate being operativelyassociated with one another such that when the contact on the irradiatedside of the diamond crystal plate is constituted as a blocking contactin relation to the charge carriers, the opposite contact is constitutedas a material capable, in conjunction with the diamond plate, ofinjecting the charge carriers under the influence of the electric fieldand the thickness of said diamond crystal plate between said contacts isless than the maximum drift length of the charge carriers created by thenuclear radiation in the diamond crystal plate, under the influence ofthe applied electric field corresponding to the limit drift rate.

2. A diamond detector according to claim 1, wherein the diamond crystalplate has a recess, the thickness of whose bottom is less than themaximum drift length of the charge carriers.

3. A diamond detector according to claim 1, wherein the blocking contactis a graphatized surface layer of a diamond crystal plate.

4. A diamond detector according to claim 1, wherein the contactinjecting holes and electrons is made from graphite.

5. A diamond detector according to claim 1 wherein the diamond crystalplate has a maximum drift length of electrons exceeding the maximumdrift length of holes, and wherein the contact of positive potential isconstituted as a material capable, in conjunction with diamond, ofinjecting holes.

6. A diamond detector according to claim 5, wherein the blocking andhole-injecting contact is silver.

7. A diamond detector according to claim 5, wherein the blocking andhole-injecting contact is gold.

8. A diamond detector according to claim 5, wherein the blocking andhole-injecting contact is platinum.

9. A diamond detector according to claim 5, wherein the blocking andhole-injecting is a surface layer of the diamond crystal plate dopedwith boron.

10. A diamond detector according to claim 5, wherein the blocking andhole-injecting is a surface layer of the diamond crystal plate dopedwith aluminum.

11. A diamond detector according to claim 1, wherein the diamond crystalplate has a maximum drift length of electrons less than the maximumdrift length of holes, and wherein the contact of negative potential isconstituted as a material capable, in conjunction with diamond, ofinjecting electrons.

12. A diamond detector according to claim 11, wherein the blocking andelectron-injecting is a surface layer of the diamond crystal plate dopedwith phosphorus.

13. A diamond detector according to claim 11, wherein the blocking andelectron-injecting is a surface layer of the diamond crystal plate dopedwith lithium.

14. A diamond detector according to claim 11, wherein the blocking andelectron-injecting is a surface layer of the diamond crystal plate dopedwith carbon.

1. A diamond nuclear radiation detector comprising a diamond crystalplate having opposite sides one of which is irradiated; contacts on theirradiated and opposite sides of said plate adapted for applying apotential difference thereacross when detecting nuclear radiations, oneof the contacts being adapted for the application of a positivepotential, whereas the opposite contact is adapted for the applicationof a negative potential, said contacts and said plate being operativelyassociated with one another such that when the contact on the irradiatedside of the diamond crystal plate is constituted as a blocking contactin relation to the charge carriers, the opposite contact is constitutedas a material capable, in conjunction with the diamond plate, ofinjecting the charge carriers under the influence of the electric fieldand the thickness of said diamond crystal plate between said contacts isless than the maximum drift length of the charge carriers created by thenuclear radiation in the diamond crystal plate, under the influence ofthe applied electric field corresponding to the limit drift rate.
 2. Adiamond detector according to claim 1, wherein the diamond crystal platehas a recess, the thickness of whose bottom is less than the maximumdrift length of the charge carriers.
 3. A diamond detector according toclaim 1, wherein the blocking contact is a graphatized surface layer ofa diamond crystal plate.
 4. A diamond detector according to claim 1,wherein the contact injecting holes and electrons is made from graphite.5. A diamond detector according to claim 1 wherein the diamond crystalplate has a maximum drift length of electrons exceeding the maximumdrift length of holes, and wherein the contact of positive potential isconstituted as a material capable, in conjunction with diamond, ofinjecting holes.
 6. A diamond detector according to claim 5, wherein theblocking and hole-injecting contact is silver.
 7. A diamond detectoraccording to claim 5, wherein the blocking and hole-injecting contact isgold.
 8. A diamond detector according to claim 5, wherein the blockingand hole-injecting contact is platinum.
 9. A diamond detector accordingto claim 5, wherein the blocking and hole-injecting is a surface layerof the diamond crystal plate doped with boron.
 10. A diamond detectoraccording to claim 5, wherein the blocking and hole-injecting is asurface layer of the diamond crystal plate doped with aluminum.
 11. Adiamond detector according to claim 1, wherein the diamond crystal platehas a maximum drift length of electrons less than the maximum driftlength of holes, and wherein the contact of negative potential isconstituted as a material capable, in conjunction with diamond, ofinjecting electrons.
 12. A diamond detector according to claim 11,wherein the blocking and electron-injecting is a surface layer of thediamond crystal plate doped with phosphorus.
 13. A diamond detectoraccording to claim 11, wherein the blocking and electron-injecting is asurface layer of the diamond crystal plate doped with lithium.
 14. Adiamond detector according to claim 11, wherein the blocking andelectron-injecting is a surface layer of the diamond crystal plate dopedwith carbon.